This invention relates to electrophotography devices, and more particularly to a cleaning and developing unit of a printing apparatus utilizing the principles of electrophotography, namely, an electronic printing apparatus.
Printing according to an electronic photographic system has been well known in the art. For instance, a xerographic printing system disclosed by U.S. Pat. No. 2,579,047 and an electronic printing apparatus disclosed by Japanese Patent Application Publication No. 1554/1968 are well known. In this prior art, a toner image is formed on a photo-conductive plate according to an electrostatic photographic method and is then fixed, so that a non-photo-sensitive insulating pattern, namely, a printing master, is formed on the photo-conductive plate. In succession, the printing master is uniformly charged and uniformly exposed to light, whereby the charges remain on only that image region which is not photo-sensitive, while the charges are removed from the photo-sensitive region. Therefore, by applying charged toner to the plate, which has been partially charged, a toner image is formed thereon. The toner image is then transferred onto a suitable image support. The above-described operation is repeatedly carried out to obtain prints.
More specifically, the aforementioned Japanese Patent Application No. 1554/1968 discloses an apparatus as shown in FIG. 1. In FIG. 1, reference numeral 1 designates a rotary drum having pawls on its cylindrical wall, to hold a photo-sensitive sheet 10. Provided around the drum 1 are a cleaning unit 8, a charging unit 4, a uniform exposure lamp 13, a developing unit 5, a transferring corotron 6 and an infrared fixing unit 7 for fixing a toner image on the photo-sensitive sheet 10.
The printing master is formed as follows. A photo-sensitive sheet 10, after being wound on the drum 1, is charged by the charging unit 4. The image of an original on a platen 3 illuminated by lamp 14 is projected onto the photo-sensitive sheet 10 through a projecting lens 2, so that a electrostatic latent image is formed therein. The latent image is developed by the developing unit 5, and the toner image is fixed on the photo-sensitive sheet 10 by the infrared fixing unit 7. Thus, a printing master is formed.
The method of obtaining a number of copies from the printing master thus formed by electrostatic printing is as follows: The printing master having the toner image on the photo-sensitive sheet is charged by the charging unit 4 and is then uniformly exposed to light with the overall exposure lamp 13. The charges on the photo-sensitive sheet 10, which is uniformly charged, are selectively discharged by the overall exposure, with the exception of those on the toner image region which is not photo-sensitive or photo-conductive. Thus, a printing master having charges on the toner-image region only is formed. The printing master thus formed is then developed by the developing unit 5. A transfer sheet 11 is delivered to the transfer section in synchronization with the rotation of the drum 1. The toner-image on the printing master is transferred onto the transfer sheet by the corotron 6 and then the image thus transferred is fixed by a fixing unit 9. Thereafter, the sheet is passed into exit tray. The toner remaining on the transfering master after printing is removed by the cleaning unit 8, to complete one printing cycle. The printing cycle as described above is repeatedly carried out as many times as the required number of copies.
The electrostatic printing apparatus as described above provides a number of copies. Therefore, a large quantity of used toner is accumulated in the cleaning unit 8, and accordingly inspection and maintenance of the cleaning unit is essential, i.e., it is necessary to frequently remove the toner from the cleaning unit. In order to solve this problem, a method is known in which a cleaning function is assigned to the developing unit 5, to thereby eliminate the cleaning unit. In the case where such a combination cleaning and developing unit is used, in order to obtain one copy, a method is used in which the drum is caused to make two revolutions to alternatively carry out developing and cleaning. Alternatively, a method is employed in which a bias voltage applied to the developing unit during the developing process is made different from that applied during cleaning.
However, the method is disadvantageous in that the apparatus for practicing the method is intricate, and that, as the drum must make two revolution for one copy, the method is not suitable for high speed copying.